In general, increase of a tire temperature in a pneumatic tire is considered unfavorable in terms of durability since the increase accelerates time-dependent changes such as a change in material property, and leads to breakage of a tread portion and the like when the tire travels at high speed. Decreasing a tire temperature has been a challenge for enhancing durability particularly in the cases of an off-the-road radial tire (ORR) and a truck/bus radial tire (TBR) which are used under heavy loads, and in the case of a run-flat tire when travelling in a punctured state (when travelling with an inflation pressure thereof at 0 kPa).
For example, in a run-flat tire having a sidewall reinforcing layer whose cross-sectional shape in a tread-width direction is crescent, when the tire travels in a punctured state, deformation in a tire radius direction concentrates on the sidewall reinforcing layer, and causes the sidewall reinforcing layer to reach a very high temperature, whereby durability is severely affected.
As other means for decreasing a tire temperature in such a pneumatic tire, particularly, a temperature in a tire side portion, there is disclosed a technology of providing a reinforcing member which reduces distortion of each of constituent members (particularly, a carcass layer located in a sidewall portion, and a bead portion) of the pneumatic tire (Japanese Patent Application Publication No. 2006-76431).
There is also known a pneumatic tire including a rim guard having a large number of ridges arranged thereon as another means for decreasing a tire temperature in a pneumatic tire, the rim guard being disposed outward, in a tread-width direction, of a bead portion which comes in contact with a rim.
However, in the case of a conventional pneumatic tire in which the above described reinforcing member is provided, provision of the reinforcing member increases a tire weight, and leads to occurrence of an unintended additional failure such as separation (detachment) of the reinforcing member. There has been a problem that normal traveling performances such as driving stability and riding comfort are deteriorated thereby. Particularly in the case of a run-flat tire, it has been feared that vertical spring (elasticity in a vertical direction of the tire) when the tire travels with a normal inflation pressure becomes high, and thereby deteriorates normal traveling performances. Therefore, a method for not damaging these normal traveling performances has been demanded.
On the other hand, in the case of the pneumatic tire in which the rim guard is provided, heat release is accelerated by increase of a surface area of the tire. However, efficient heat release of a tire side portion has not been achieved only by the simple increase of the tire surface area since a rubber member with low heat conductivity is arranged in the outer circumferential side of the pneumatic tire.
Therefore, the present invention was made in consideration of the above described situation, and aims to provide a pneumatic tire capable of enhancing durability by decreasing of a tire temperature, specifically a temperature inside a tire side portion, through efficient heat release.